The present invention relates to a method and an arrangement for discharging an energy storage system for electrical energy, particularly in a vehicle having a hybrid drive line (hereinafter referred to as “hybrid vehicle”), by means of a discharge resistor as well as to a hybrid vehicle comprising such an arrangement.
In the following the invention is described in connection with hybrid vehicles. However, the invention is not limited to this application. For instance, it can be also used in stationary power generation systems comprising such an energy storage system. The invention is also applicable to stationary and/or movable construction equipments such as construction vehicles or off road machines.
Hybrid vehicles in general comprise at least two drive engines, wherein a traditional combustion engine is most frequently supported by an additional electrical drive unit. The electric energy for running the electric drive unit is commonly provided by an energy storage system like high voltage capacitors or batteries, wherein the electric energy stored therein is produced by a fuel cell or a generator.
In case such a hybrid vehicle is involved in an accident there is a potential hazard to a rescue team or other assisting persons or to the driver and the passengers themselves due to the high voltage of the energy stored in the energy storage system. Therefore, the energy storage system has to be discharged in such situations as fast as possible.
In general, discharging a high voltage energy storage system is preferably performed by a discharge resistor. The discharge resistor limits the discharging current and prevents the energy storage system from exploding or behaving in an uncontrolled dangerous way during the discharge process or thereafter. The resistance of the discharge resistor is chosen to be large enough to keep control of the discharging process and to discharge the energy storage system without damaging it.
Another discharge process is disclosed in the Japanese patent application JP 2004129367 wherein two discharge methods (discharge by means of an usual discharge resistor and discharge by corona discharge) are performed in parallel. The discharge process itself is initialized in case of an accident.
Disadvantageously, the use of a large resistance for the discharge resistor or the use of the two discharge methods described above (even if they are performed in parallel) means that the discharge process takes several minutes before it is completed. During that time no safe rescue operation can be performed which might even cost a person's life.
It is desirable to provide a method and an arrangement for discharging an energy storage system for electrical energy that reduces the time needed for discharging the energy storage system. It is also desirable to provide a method and an arrangement that reduces the risk exposure of the environment for hazardous side effects caused by the discharge process.
A method and an arrangement for discharging an energy storage system for electrical energy (hereinafter generally referred to as “energy storage system”), as well as by a hybrid vehicle comprising such an arrangement are disclosed.
The invention is based, according to an aspect thereof, on the conclusion that one of the major limiting factors for using a resistor with a rather low resistance for discharging an energy storage system is the heat generated during the discharge process. Therefore, according to an aspect of the invention a coolant is provided at the resistor during the discharge process for leading off the heat produced during the discharge process. Advantageously, the discharge resistor can be made of an alloy having a positive temperature coefficient, such as canthal, constantan or tungsten.
In addition to the fact that its price is rather low, the use of carbon dioxide gas (CO2) as coolant has the advantage that carbon dioxide gas also serves as fire extinguisher. In case the resistor and/or the energy storage system starts burning because of the heat developed by the fast discharge, the coolant also could stop, or prevent, the fire. Therefore, in a preferred embodiment of the invention the coolant is also provided at the energy storage system itself.
Further, in a preferred embodiment, the coolant is stored under pressure in a pressurized storage unit. Releasing the coolant results in a sudden drop of temperature of the storage unit. The sudden drop in temperature can also be used for further cooling down the energy storage system and/or the discharge resistor. This effect can be maximized by integrating the storage unit into the energy storage system and/or the discharge resistor. Integrating the storage unit has the further advantage that the arrangement size can be reduced.
Another advantageous embodiment uses at least one energy consumer being in connection with the energy storage system for discharging the energy storage system and/or to support the discharge process. In case a vehicle energy storage system needs to be discharged, the use of an electrical engine for discharging the energy storage system is preferred.
In another preferred embodiment, the initializing of the discharge process is triggered by a trigger signal. The trigger signal can be transmitted automatically for example by the vehicle and/or manually for example by a remote control operated by a rescue team.
Preferably, the trigger signal is transmitted by an accident sensing sensor or such system like an anti-collision detection system. The accident sensing sensor, or system, can be a sensor for realizing an accident the moment it happens, correlating the initialization of the discharge process for example to an airbag deployment signal, or an accident prediction sensor calculating the probability of an accident and transmitting the trigger signal in case the probability exceeds a certain threshold. Both solutions have the main advantage that the discharge is already in process when the rescue team arrives or is, in case of the accident prediction sensor, or system, already completed or almost completed.
It is further advantageous to provide a signal possibility signaling the status of the discharge process, i.e. that the discharge of the energy storage system is completed, or is still in process and/or still needs to be initiated and performed.
Further advantages and preferred embodiments are disclosed in the figures and the description.